Infant sleep machines have become a popular aid for parents with a baby getting inadequate sleep because of a busy household, loud neighbors or other noise disturbances.

But a new study says parents should be cautious with the devices because they can generate sound levels that could place infants at risk of developing noise-induced hearing loss. In addition, they are often sold with limited or no instructions for safe use, says the study published online Monday by the journal Pediatrics.

The machines -- which can be used to mask environmental noises or provide ambient noise designed to soothe an infant during sleep -- "are capable of producing levels that may be damaging to babies' hearing," says Blake Papsin, otolaryngologist-in-chief at the Hospital for Sick Children in Toronto and senior author of the study.

Using a sound level meter, Papsin and colleagues tested the maximum noise levels of 65 sounds in 14 different infant sleep machines when placed at three distances: 30 centimeters (11.7 inches -- simulating placement on a crib rail): 100 centimeters (39 inches -- simulating placement on a table near a crib); and 200 centimeters (78 inches -- simulating placement across the room from a crib.) Calculations were added to account for the size and development of a 6-month-old's ear canal.

-- All but one machine exceeded that recommended noise limit even when placed across the room, 200 centimeters away.

--Three machines produced outputs greater than 85 decibels when placed 30 cm away. If played continuously, as recommended on several parenting websites, infants would be exposed to sound pressure levels that exceed the occupational noise limits for an 8-hour period endorsed by the National Institute for Occupational Safety and Health and the Canadian Centre for Occupational Health and Safety

Regular exposure to white noise through an infant sleep machine on a nightly basis is of particular concern, Papsin says, because an infant's brain needs the stimulation it receives from a range of sounds in order to develop properly. "Completely removing all informational content at a loud, potentially damaging level is the worst," he says.

Most of the devices studied featured a volume control, which suggests that safer use is possible and that the machines can be used in accordance with recommendations for hospital nursery noise, Papsin says.

With that in mind, the study says manufacturers should be required to limit maximum output levels; print warnings about noise-induced hearing loss on the machine's packaging; and include a mandatory timer on machines marketed primarily for infants that would make them automatically shut off after a predetermined period of time.

Families can more safely use the machines if they place them as far away as possible from the infant and never in the crib or on a crib rail; play the the devices at a low volume; and operate them for a short duration of time.

But this research highlights the potential for a previously "unknown harm that can occur," Yuan says. "We're getting more and more concerned about issues related to sound and noise and hearing-loss in children because it's progressive."

Given today's very industrial lifestyle, "there's a lot of noise around, so it's always good, especially with young children, to really know what types of exposures they're getting and the effects they can have long term," adds Linda Hazard, an audiologist certified by the American Speech-Language-Hearing Association ​ and director of the Vermont Early Hearing Detection and Intervention Program.

"It's important to recognize that the authors are not saying not to use (the machines)," says Hazard, who was not involved in the study. "They're saying use them judiciously and get information about how they're performing." The recommendation that manufacturers include instructions about how the devices should be used is equally important, she says.

The paper "intentionally" does not identify any specific brands, models or manufacturers, Papsin says, "because the point is to start the conversation that every one of these is capable of damage if used inappropriately."